The invention relates to the field of communication electronics, and, more particularly, to multipath mitigation in spread spectrum communications used in a wireless communications network.
Wireless or radio communication between separated electronic devices is widely used. For example, a wireless local area network (WLAN) is a flexible data communication system that may be an extension to, or an alternative for, a wired LAN within a building or campus. A WLAN uses radio technology to transmit and receive data over the air, thereby reducing or minimizing the need for wired connections. Accordingly, a WLAN combines data connectivity with user mobility, and, through simplified configurations, also permits a movable LAN.
Over the past several years, WLANs have gained acceptance among a number users including, for example, health-care, retail, manufacturing, warehousing, and academic areas. These groups have benefitted from the productivity gains of using hand-held terminals and notebook computers, for example, to transmit real-time information to centralized hosts for processing. Today WLANs are becoming more widely recognized and used as a general purpose connectivity alternative for an even broader range of users. In addition, a WLAN provides installation flexibility and permits a computer network to be used in situations where wireline technology is not practical.
In a typical WLAN, an access point is provided by a transceiver, that is, a combination transmitter and receiver, connects to the wired network from a fixed location. Accordingly, the access transceiver receives, buffers, and transmits data between the WLAN and the wired network. A single access transceiver can support a small group of collocated users within a range of less than about one hundred to several hundred feet. The end users connect to the WLAN through transceivers which are typically implemented as PC cards in a notebook computer, or ISA or PCI cards for desktop computers. Of course the transceiver may be integrated with any device, such as a hand-held computer.
Spread spectrum communications have been used for various applications, such as cellular telephone communications, to provide robustness to jamming, good interference and multi-path rejection, and inherently secure communications from eavesdroppers, as described, for example, in U.S. Pat. No. 5,515,396 to Dalekotzin. The patent discloses a code division multiple access (CDMA) cellular communication system using four Walsh spreading codes to allow transmission of a higher information rate without a substantial duplication of transmitter hardware. U.S. Pat. No. 5,535,239 to Padovani et al., U.S. Pat. No. 5,416,797 to Gilhousen et al., U.S. Pat. No. 5,309,474 to Gilhousen et al., and U.S. Pat. No. 5,103,459 to Gilhousen et al. also disclose a CDMA spread spectrum cellular telephone communications system using Walsh function spreading codes. The disclosures of each of these patents is hereby incorporated by reference in their entirety.
Multipath is one major problem in spread spectrum communications, especially within the use of WLAN systems. The building environment in which a WLAN is placed typically acts as a echo chamber because the walls, doors and furniture cause the spread spectrum signals to arrive at a receiver at different times. These signals can add and subtract and cause difficult multipath problems. Although antenna diversity is often used to mitigate multipath, once an antenna is chosen, it is also necessary to mitigate the effects of multipath within the base band processor itself.
It is therefore an object of the present invention, to mitigate multipath within the base band processor as an improvement over prior art techniques.
In accordance with the present invention, a demodulator for a spread spectrum radio transceiver allows multipath mitigation and includes a demodulator circuit for spread spectrum phase shift keying (PSK) demodulating an information signal received from a radio circuit. The information signal includes data symbols formed from a plurality of high rate mode chips forming a spread spectrum information signal. The demodulator circuit further includes a signal input for receiving the information signal and at least one predetermined code function correlator in line with the signal input for decoding the information signal according to a predetermined code. A carrier loop circuit allows phase and frequency tracking of the information signal. A chip decision circuit is operative with the carrier loop circuit for tracking high rate mode chips.
In accordance with the present invention, an equalizer is included and has a feed forward finite impulse response filter positioned in line to the code function correlator and the signal input and has a plurality of feed forward taps. At least one feed forward tap is selected for logical multiply operations to aid in reducing multipath signal echoes.
In accordance with another aspect of the invention, a feedback finite impulse response filter could be used with or without the feed forward finite impulse response filter. The feedback finite impulse response filter is operative with the chip decision circuit and the carrier loop circuit and has a plurality of feedback taps. At least one feedback tap is selected for logical add/subtract operations to aid in canceling signal echo.
In accordance with one aspect of the present invention, at least two feed forward taps are selected, and in still another aspect, at least three feed forward taps are selected. The demodulator also includes means for selecting the feed forward taps before selecting feedback taps. The at least one feedback tap can be selected based on the channel impulse response. Feedback taps can be set substantially equal to the channel impulse response of one sample per chip. In still another aspect of the present invention, the code function correlator comprises a Walsh code correlator. The carrier loop circuit also includes a carrier numerically controlled oscillator (NCO), and a carrier NCO control means for selectively operating the carrier NCO based upon a carrier phase on the carrier loop circuit.
In still another aspect of the present invention, the spread spectrum radio transceiver of the present invention includes a base band processor and a radio circuit connected thereto. The base band processor includes a demodulator for spread spectrum phase shift keying (PSK) demodulating an information signal received from a radio circuit. The information signal includes data symbols formed from a plurality of high rate mode chips forming a spread spectrum information signal. The demodulator further includes a signal input for receiving the information signal and at least one predetermined code function correlator in line with the signal input for decoding the information signal according to a predetermined code.
A carrier loop circuit allows phase and frequency tracking of the information signal. A chip decision circuit is operative with the carrier loop circuit and tracks high rate mode chips. An equalizer includes a feed forward finite impulse response filter positioned in line to the code function correlator and the signal input and has a plurality of feed forward taps. At least one feed forward tap is selected for logical multiply operations to aid in reducing multipath signal echoes. A feedback finite impulse response filter is operative with the chip decision circuit and carrier loop circuit and has a plurality of feedback taps. The at least one feedback tap is selected for logical add/subtract operations to aid in canceling multipath signal echoes. A modulator spread spectrum PSK modulates information for transmission via the radio circuit. The modulator comprises at least one code function encoder for encoding information according to a code sequence.
A method aspect of the invention is also disclosed. In accordance with the present invention, a method of demodulating a spread spectrum signal comprises the steps of receiving a spread spectrum phase shift keyed (PSK) modulated signal within a demodulator of a spread spectrum receiver. The method also comprises equalizing the signal with a finite impulse response filter operatively connected to a chip decision circuit and carrier loop circuit by selecting at least one feedback tap for logical add/subtract operations to aid canceling multipath signal echoes. The method can also comprise the step of equalizing the signal with a feed forward finite impulse response filter having selective feed forward taps and which is positioned in line to a code function correlator for a multiply logic operation. The method can also comprise the step of selecting at least two feed forward taps and at least three feed forward taps. The feed forward taps are typically selected before feedback taps.